The present invention relates to in vivo analysis of blood flow.
Recent advances in methods of studying blood flow have provided a relatively untapped resource for measuring parameters that have previously been either indeterminate or accessible only via invasive procedures. Whether these techniques use fluorescent lipsomes, microspheres, or labeled leukocytes or erythrocytes, they generally provide measures of particle position versus time; by comparing particle positions from successive video frames. One particular application of this technique is in the eye, where particle movements are used to obtain measures such as relative ocular blood flow.
While the foregoing techniques have been diagnostically useful, diagnosis of many conditions requires accurate measurement of absolute blood flow rates in areas of interest. For example, in examining ocular tissue such as the retina choroid or optic nerve head, absolute blood flow rates in capillaries can be critical in diagnosing the regulation of diseases Such as diabetes and hypertension, by providing a measurement of the dilation of veins. Absolute blood flow rate measurements are also useful in diagnosing closures of small capillaries (known as capillary dropout) and microaneurisms.
Using a scanning laser ophthalmoscope (SLO), fluorescent particles can be readily imaged in the retinal and choroidal blood vessels. However, using current techniques, the motion of particles cannot be associated with absolute blood flow rates, limiting the diagnostic usefulness of these techniques.
Accordingly, there is a need for a method of measuring absolute flow rates in passages such as retinal and choroidal blood vessels, or the entire retina and choroid, which is automated and objective.
In accordance with principles of the present invention, this need is met by new techniques for quantifying absolute blood flow. In the specific embodiment described herein, absolute volumetric blood flow is quantified using digitized video frames of fluorescent particles circulating in the retina and choroid.
In accordance with principles of the present invention, a particle counting method is used upon a subject having a known blood concentration of particles. A first step of the method is to obtain a measure of the blood concentration of particles in the subject. This measure can then be used to determine absolute blood flow, by monitoring the passage of particles in a blood vessel of interest over a period of time. A measure of absolute flow in regions of the blood vessel is then generated based upon the number of particles passing through that region over the period of time, combined with the blood concentration of particles.
In various disclosed embodiments, the measure of the blood concentration of particles may be obtained by direct measurement, i.e., by withdrawing a sample of the subject""s blood and counting particles within a unit volume of the sample. As long as the particle diameters are sufficiently small compared to the blood vessels and the particles are uniformly distributed in the blood, it may be assumed that the number of distinct particles recorded in a particular vessel is proportional to the amount of blood flowing within it, and that each particle represents a given volume of blood as determined by the measured blood concentration of particles.
A measure of the number of particles passing through a vessel of interest is obtained by counting particles within the vessel during successive video frames. The counts of particles are accumulated. This count can then be converted to an amount of blood flow using the measured blood concentration of particles.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.